1. Field of the Invention
The present invention relates to a dry film photoresist and more particularly, to the dry film used for the manufacture of PCB (printed circuit board; hereinafter referred to as "PCB"), lead frame and BGA (ball grid array), comprising a polyethylene terephthalate film to satisfy some surface properties as a cover film, thus minimizing any defective fraction of a product requiring a higher resolution.
2. Discussion of Related Art
Since its development by Dupont of U.S.A. in the brandname of "RISTON" in 1968, a dry film photoresist, i.e., a photosensitive film, has been employed as one of the vital elements in the electric/electronic field, particularly in the PCB processing.
Now that PCB's are widely applied as a basic part of the contact circuit in some household electronics, including industrial electronic equipments such as a computer and electrocommunication equipments, the film photoresist has been an epoch-making processing material involved in the manufacture of such industrial equipment.
About 50 percent of all photoresist materials designed for circuit formation on the PCB has been occupied by a photosensitive screen printing ink and liquid photoresist. While the film photoresist has been applied by necessity manufacturing the PCB with both-side plate and multi-layer plate requires a higher density and reliability.
The dry film photoresist consists of three layers, i.e., a base film, a photosensitive layer and a cover film.
In general, as far as a base film is concerned, a polyester film with the thickness of 16 .mu.m, 19 .mu.m, 25 .mu.m derived from polyethylene terephthalate has been employed.
In the process of fabricating a dry film photoresist, a polyester base film is employed as a supporter, while a cover film serves to prevent a damage of resist by dusts and handling thereof.
Further, a photosensitive layer coated on a polyester film has the thickness ranging from 15 to 100 .mu.m to adequately meet the purposes of use. The photosensitive layer has various compositions depending on the mechanical/ chemical characteristics of a film photoresist and processing conditions.
More specifically, the photosensitive layer comprises a) a multifunctional monomer whose photopolymerization is induced by light, b) a radical-inducing photoinitiator by light so as to bring about the photopolymerization, c) a binder polymer providing mechanical strength, tenting property and adhesiveness of a photopolymerization composition, and d) some additives such as dyes, stabilizers, adhesion promoter or thermal polymerization inhibitor. These materials, dissolved in an appropriate solvent, are coated on a polyester base film and then dried.
The photosensitive polymer may be used as a liquid phase itself, while the photosensitive layer consisting of photohardening components may be also used by lamination between a photo-permeating polyester film and cover film to secure a better workability and anti-contamination.
Referring to each component in more detail, the multifunctional monomer, photopolymerized by an initiator, shall have a resistance to a developing solvent during the post-process, while being removed by a stripping ingredient.
The commonly available multifunctional monomer include a compound having one or more .alpha.,.beta.-ethylenic unsaturated bonds. Further, according to the photopolymerization, a compound having two or more acryloyl groups or methacryloyl group in the molecule is preferred as a multifunctional monomer. Examples of such compound include glycol diacrylate derivatives such as ethylene glycol diacrylate, diethylene glycol diacrylate, triethylene glycol diacrylate, polyethylene glycol diacrylate, propylene glycol diacrylate, dipropylene glycol diacrylate, polypropylene glycol diacrylate, 1,4-butanediol diacrylate, 1,5-pentanediol diacrylate or 1,6-hexanediol diacrylate; additive material of ethylene or propylene oxide of Bisphenol A, such as N,N'-methylene bisacrylamide or N,N'-benzylidene bisacrylamide; and, some compounds having three or more acrylates such as glycerin triacrylate, trimethylol propane triacrylate, trimethylol ethane triacrylate, pentaerythriotol triacrylate, pentaerythritol tetracrylate, dipentaerythriotol hexacrylate, or methacrylate compounds threrof.
The photoinitiator, which functions to excite electrons by ultraviolet rays, plays a role to generate the radical or induce other compounds to generate the radical, thus contributing to the polymerization of multifunctional monomer. The photohardening rate of each component varies greatly depending on the types and contents of photoinitiator. Detailed examples of the photoinitiator include benzoin, benzoin methylether, benzoin ethylether, benzoin isopropylether or benzyl; alkylbenzophenone such as benzophenone, 4,4'-bis(diethylamino)benzopheone, chlorobenzophenone, 4,4'-dimethylaminobenzophenone or 4,4'-dichlorobenzophenone; anthraquinones such as 2-ethylanthraquinone or 2-t-butylanthraquinone; and, 4-(dialkylamino)benzoic acid alkylester or 2,4,5-triarylimidazle dimer and its derivatives or lobine dimer, or dyes such as leucotriphenylmethane, triarylmethane leuco-dye, or a composite containing a dye and lobine dimer.
Meantime, examples of the binder polymer include organic polymers such as acrylic polymers, styrene polymers, polyvinyl acetate, ethylene vinyl acetate, polyvinyl chloride, vinyl chloride-vinyl acetate copolymer, chlorinated polyethylene, chlorinated polypropyrene, vinylidene chloride polymers, polyester, polyamide, polyurethane, polyvinyl acetal, alkyd resin, phenol resin, epoxy resin, acetic acid cellulose, nitrificated cotton, styrene-butyrene copolymer, styrene-acrylonitrile copolymer, vinyl copolymer of chlorinated rubber or maleic anhydride, and aromatic sulfonamide formaldehyde resin.
Examples of the additives include dyes, stabilizers, adhesion promoters and thermal polymerization inhibitors.
Typical examples of leuco-dye include methyl, malachite green and crystal violet, while other dyes may be added depending on the purposes of use.
In general, a polyolefin film having the thickness ranging from 25 to 30 .mu.m is employed as a cover film to fabricate the dry film photoresist; such polyolefin film, rolling the adhesive photosensitive layer to an axle without adhesion, plays an important role in the manufacture, transportation and storage of a film photoresist.
When a dry film photoresist with the above structure is to be employed, a stripped cover film is laminated on a copper-laminated plate, exposed by the ultraviolet rays and developed to form the image.
More specifically, a dry film photoresist composition is imbedded on the surface of a metal by coating or lamination and then, it is subject to exposure by ultraviolet rays. The desired image is formed in such a manner that the exposed area is hardened, while the unexposed area is removed with suitable solvents.
The solvents are largely classified into water-soluble and water-insoluble ones.
Since the water-insoluble solvents are blamed for adverse working conditions, environmental contamination and high production cost, use of photohardening compositions developed by water-soluble solvents have been on the gradual increase.
In the alkali-developing photosensitive resist, so prepared from the composition of a dry film photoresist, a cover film layer is removed from the dry film photoresist. Then, it is laminated in a copper plate and exposed by ultraviolet rays using a photomask film. The unexposed area is removed with an aqueous solution of alkali at the given concentration and temperature to form a desired photoresist pattern.
The term `developing` refers to a process in which any unexposed area is removed using an aqueous solution of alkali to form a desired photoresist pattern. During the developing process, a photosensitive layer at the unexposed area is dissolved in an aqueous solution of sodium carbonate or potassium carbonate (0.5-1.5 wt. %) at the temperature ranging from 25 to 30.degree. C., thus forming an image.
The fabricating method of the PCB is divided into a plating method and tenting method. According to the plating method, when a dry film photoresist is plated on a substrate by some plating solution and various additives, its anti-plating property is required so as not to be stripped by erosion. In contrast, the elasticity itself in the membrane of a dry film photoresist is required under the tenting method.
Meantime, the dry film photoresist functions as a hole-covering resist to protect from an etching solution some holes with certain size, serving to provide the conductivity to the upper and lower sides of a substrate.
Hence, the hole-covering dry film in a substrate shall have a sufficient elasticity so as to endure an outer stress under the developing conditions according to a spraying method of sodium carbonate or potassium carbonate in an aqueous solution (0.5-1.5 wt. %) at 25 to 35.degree. C. and under the etching conditions according to a spraying method of FeCl.sub.2, FeCl.sub.3, CuCl.sub.2, CuCl.sub.3 and ammonia persulfate at 45 to 55.degree. C.
However, the conventional alkali-developing photosensitive film as a cover film contains a polyolefin film such as polyethylene.
With some physical properties such as better flexibility, chemical-proofing and mold releasing, the polyethylene film may be easily removed, in case of using as a cover film of photosensitive resin composition. Further, since its friction coefficient on polyethylene terephthalate is large, little telescope (an original form of product is not maintained, while a foam comes out) does exist during the fabrication of a final product.
Nevertheless, during polymerization of polyethylene macromolecular gel is generated and after a film is formed, some micro-protuberances like fish eyes occur.
These fish eyes from the elongated polyethylene film may have a size of over 1 mm; some fish eyes having a size of over 0.3 mm are generated in the range from 500 to 50,000 particles/m.sup.2, while numerous fish eyes having a size of less than 0.3 mm are generated.
When these fish eyes are radiated into a photosensitive layer for lamination, the air admixture is induced. In particular, when the thickness of a photoresist is thinner, the probability of product defect becomes higher.
FIG. 1 is a schematic view illustrating the air admixture generated by fish eyes.
In case where a polyethylene film 13 with a fish eye 20 is laminated on a copper laminated plate containing an epoxy resin 32 and a copper 31, which is radiated into the photosensitive layer 12, induces the air admixture at the side laminated with the copper plate.
The magnitude of air admixture will vary depending on the width and height of fish eye. Also, the thinner a photoresist in thickness is, the greater such magnitude of air admixture becomes.
In addition to the above factors, the air admixture may be generated by factors such as a poor substrate having holes, deep scratching trace, traces pressed by foreign materials present in a photoresist, and damages or scratch of laminator roll.
As shown in FIG. 1, the air admixture is not entirely involved in the product defects, and these defects may occur depending on certain circumstances. When the air-admixture area is located at the border of the non-exposed area, a circuit is reduced in the innerlayer process which may result in a circuit loss; in a plating process, the air admixture is responsible for expanding a circuit which may result in a short circuit.